Hello there, reecewallis.
The following Knowledge Base article provides some great information in regards to AirPlay and playing your music through multiple sources:
iTunes: About playing music with AirPlay
http://support.apple.com/kb/HT4325
With AirPlay and iTunes 10 or later, you can listen to music through your computer speakers and multiple AirPlay outputs.
The maximum number of outputs is three to six. The number that works for you depends on your environment (such as building composition and local radio interference), distance to the outputs, and available network capacity relative to your other usage.
You can use the Multiple Speakers window to control the volume of your output devices.
The sound from your AirPlay output device may seem louder or softer because of the acoustics in the area, and the sound may be different compared to the sound from the same device in a different area.
You can adjust Master Volume in the Multiple Speakers window or use the main volume control in the upper-left corner of the iTunes window. If you set the softest output to match the Master Volume, you have more precise control over that output when you adjust the Master Volume. You can set loud AirPlay outputs to a lower volume to better control the extra power.
Thanks for reaching out to Apple Support Communities.
Cheers,
Pedro.
